1) Field of the Invention
The present invention relates to an optical transmission apparatus (optical add/drop apparatus) and an optical transmission system suitable for use with a WDM (Wavelength Division Multiplexing) optical transmission system such as, for example, a high-capacity long-distance submarine optical transmission system.
2) Description of the Related Art
Among WDM optical transmission systems which are being developed at present, for example, those systems (DWDM) which perform a wavelength division multiplexing process for 60 to 150 signal lights of different waves are being popularized in order to implement a high-capacity system.
For example, as shown in FIG. 6, a high-capacity long-distance optical submarine transmission system (optical submarine cable system) 100 as a DWDM optical transmission system includes a transmitting station (terminal) 101 for transmitting wavelength division multiplexed signal lights, and an optical transmission line (trunk line) 102 for interconnecting the transmitting station 101 and a receiving station 103. The receiving station (terminal) 103 receives the wavelength multiplexed signal lights transmitted thereto through the optical transmission line 102. The high-capacity long-distance optical submarine transmission system 100 further includes an optical add/drop apparatus (branching unit) 104 for branching a signal light having a specific wavelength from among the wavelength division multiplexed signal lights from the transmitting station 101 and multiplexing a signal light transmitted through an optical transmission line (branch line) 106 connected to a branch station 105. The branch station (terminal) 105 receives the signal light of the specific wavelength branched by the optical add/drop apparatus 104 and transmits a signal light to be multiplexed with the signal lights transmitted in the optical transmission line 103 (for example, refer to Japanese Patent Laid-Open No. 10-28106). It is to be noted that a plurality of optical amplifiers 107 are provided on the optical transmission lines 102 and 106.
In such an optical transmission system as just described, some signal lights b from among all signal lights a (refer to reference character A) transmitted from the transmitting station 101 are branched (dropped) by the optical add/drop apparatus 104 and transmitted to the branch station 105 through the optical transmission line 106 (refer to reference character B). On the other hand, the remaining signal lights c (refer to reference character C) are multiplexed with (added to) signal lights d (refer to reference character D) transmitted from the branch station 105 by the optical add/drop apparatus 104, and the multiplexed signal lights e (refer to reference character E) are transmitted to the receiving station 103 through the optical transmission line 102.
It is to be noted that, while only the trunk line (up line or down line) for one direction is shown in FIG. 6 in order to facilitate understanding, actually, not only the one-direction trunk line but also a trunk line (down line or up line) for the opposite direction are provided so that bidirectional communication can be performed. Further, while the transmitting station 101, receiving station 103 and branch station 105 here are described as terminals, even if they are formed as repeater stations, they operate similarly. In this instance, the transmitting station 101 functions as a repeater station on the transmitting side and the receiving station 103 functions as a repeater station on the receiving side, and the branch station 105 functions as a repeater station on the branch side.
In the optical add/drop apparatus (add drop branching unit) used in the optical transmission system configured in such a manner as described above, for example, as shown in FIG. 5, an optical circulator 111, an optical fiber grating 112, an optical isolator 113, another optical fiber grating 114, and another optical circulator 115 are interposed in order in an up line (trunk line) 110. Further, an optical circulator 121, an optical fiber grating 122, an optical isolator 123, another optical fiber grating 124, and another optical circulator 125 are interposed in order in a down line (trunk line) 120 opposed to the up line 110. Further, a dropping branch line 116 is connected to the optical circulator 111, and an adding branch line 117 is connected to the optical circulator 115. Meanwhile, a dropping branch line 126 is connected to the optical circulator 121, and an adding branch line 127 is connected to the optical circulator 125 (refer to, for example, Japanese Patent Laid-Open No. 10-150433).
Thus, wavelength division multiplexed signal lights transmitted from the transmitting station through the up line 110 pass the optical circulator 111 and enter the optical fiber grating 112. In the optical fiber grating 112, only those signal lights individually having specific wavelengths (for example, approximately 20 signal lights) from among the wavelength division multiplexed signal lights (for example, 100 wavelength division multiplexed signal lights) transmitted from the transmitting station are reflected. Then, the reflected specific wavelength signal lights are sent to the branch line 116 through the optical circulator 111, and are transmitted to the branch station through the branch line 116. Consequently, the signal lights individually of the specific wavelengths (for example, approximately 20 signal lights) from among the wavelength division multiplexed signal lights (for example, 100 wavelength division multiplexed signal lights) transmitted from the transmitting station are dropped.
On the other hand, the signal lights having passed through the optical fiber grating 112 without being reflected enter the optical fiber grating 114 through the optical isolator 113. Meanwhile, signal lights transmitted from the branch station through the branch line 117 enter the optical fiber grating 114 through the optical circulator 115 and are reflected by the optical fiber grating 114. Then, in the optical fiber grating 114, the signal lights transmitted from the branch station through the branch line 117 are multiplexed with (added to) the signal lights transmitted from the transmitting station through the up line 110. The multiplexed signal lights pass the optical circulator 115 and are transmitted to the receiving station through the up line 110.
Similarly, wavelength division multiplexed signal lights transmitted from the transmitting station through the down line 120 pass the optical circulator 121 and enter the optical fiber grating 122. By the optical fiber grating 122, only those signal lights individually having specific wavelengths (for example, approximately 20 signal lights) from among the wavelength division multiplexed signal lights (for example, 100 wavelength division multiplexed signal lights) transmitted from the transmitting station are reflected. Then, the reflected specific wavelength signal lights are sent to the branch line 126 through the optical circulator 121 and are transmitted to the branch station through the branch line 126. Consequently, the signal lights individually having specific wavelengths (for example, approximately 20 signal lights) from among the wavelength division multiplexed signal lights (for example, 100 wavelength division multiplexed signal lights) transmitted from the transmitting station are dropped.
On the other hand, the signal lights having passed through the optical fiber grating 122 without being reflected enter the optical fiber grating 124 through the optical isolator 123. Meanwhile, signal lights transmitted from the branch station through the branch line 127 enter the optical fiber grating 124 through the optical circulator 125 and are reflected by the optical fiber grating 124. Then, by the optical fiber grating 124, the signal lights (for example, approximately 20 signal lights) transmitted from the branch station through the branch line 127 are multiplexed with (added to) the signal lights transmitted from the transmitting station through the down line 120. The multiplexed signal lights pass the optical circulator 125 and are transmitted to the receiving station through the down line 120.